1. Technical Field
The present invention relates to pointing devices in general, and particularly to a pressure sensor type pointing device for moving a cursor on a computer screen.
2. Description of Related Art
A pointing device of a computer generates a signal for moving a mouse pointer (cursor) displayed on a screen according to a user's operation. In general, pointing devices include a pointing stick as well as a mouse and a touch-pad. The pointing stick is also called Track Point® and is provided among keys on a keyboard. The pointing stick is mainly employed in a laptop personal computer (laptop PC) because the pointing stick can be operated with fingers in a home position. The pointing stick requires no operation space unlike the mouse, and it is easy to operate even while keeping a computer on the user's lap in a car or a train.
A pressure sensor type and a strain gauge type are known as detection principles of the pointing stick. For example, in one prior art, pressure is applied to four pressure sensors placed in a cross shape via a plate tiltable about a support rod to generate a signal corresponding to the direction and magnitude of moving a cursor. The center of a button is recessed like a dish so that, when a finger is slid horizontally in any direction while placing the fingertip in the center of the button, the finger will naturally presses peripheral parts of the button to enable an operation with a feeling of moving the button horizontally rather than pressing the button.
Another prior art discloses a pointing device in which a force in a horizontal direction is applied to an operation lever to change the resistance of four pressure-sensitive rubber materials arranged around the operation lever in a cross shape in order to detect the direction and magnitude of the force. Yet another prior art discloses a pointing device in which a force is applied to a finger operating part from above in a position deviated from an axial line, where the finger operating part is coupled to the top of a stem supported by a ball bearing, to obtain a signal corresponding to the moving direction and moving speed of a cursor by means of piezoelectric elements arranged in four directions.
Since many strain gauge type pointing sticks have been adopted so far, users are familiar with the operational feeling. FIG. 13 is a sectional view for describing the structure of a strain gauge type pointing stick 10 employed in a laptop PC. A membrane sheet 25 with key switches formed thereon is laminated on the upper surface of a metal plate 27. Multiple keys 21, 23 and the pointing stick 10 are fixed to the metal plate 27. The pointing stick 10 includes a ceramic operation post 13, a cap 11 attached to the operation post through an intermediate member 15, and a printed circuit board (PCB) 17 on the reverse side of which a strain gauge is pasted in an X direction and a Y direction.
The cap 11 is formed out of a soft material such as silicon rubber. The PCB 17 mounts thereon an operational amplifier, a processor, and the like to convert a change in electrical resistance of the strain gauge into voltage, and further to generate a digital signal corresponding to the moving direction of a cursor and the amount of travel per unit time according to the direction and magnitude of a force applied to the operation post 13. The PCB 17 is covered with a shield cover 19.
In the pointing stick 10, the crown of the cap 11 as an operation surface is flush with or projects slightly beyond the faces of the keys 21 and 23. Since the cap 11 is surrounded by three keys, including the keys 21 and 23, only the crown of the cap 11 becomes the finger-operating surface. When the operation surface of the cap 11 is applied with a force including a vertical component needed to obtain a frictional force and a horizontal component to determine the moving direction, the PCB 17 is distorted. At this time, the mouse cursor can be moved in the direction of the horizontal component even if the finger applies the force to any position on the operation surface.
Since the height of the cap 11 affects the thickness of a laptop PC, the pointing stick 10 is required to reduce the height from the shield cover 19 to the crown of the cap 11 in order to make the laptop PC thinner. However, since there is a need to apply a greater force to the cap 11 to give a predetermined strain as the operation post 13 and the cap 11 are shortened, the operability is degraded. Even if the signal amplification factor is increased to operate with a weak force, it is limited due to the influence of noise. On the other hand, even if the thickness of the PCB 17 is reduced to make the PCB 17 flexible, this is also limited because the PCB 17 becomes fragile.
Even when the pressure sensor type is employed, it is desired that the operational feeling should be equivalent to that of the strain gauge type. However, since a force in the vertical direction is applied to a pressure sensor positioned in the moving direction of the mouse cursor in the above-mentioned methods, there is a need to select a depressing position on the operation surface. Therefore, the operational feeling becomes different from the strain gauge type pointing stick 10 that moves the mouse cursor in the direction of the horizontal component regardless of the depressing position. Further, the method of applying a force in the horizontal direction to a side face of the operation lever cannot make the laptop PC thinner because of the need to lengthen the operation post so that it will project from keycaps.
Consequently, it would be preferable to provide a pressure sensor type pointing device capable of furnishing an operational feeling similar to a strain gauge type pointing stick.